The general aims of the program are to analyze genetic and epigenetic mechanisms underlying normal mammalian differentiation; and to understand the developmental basis for malignancy and for genetic diseases, particularly those involving specific biochemical lesions, as in certain human heritable syndromes. These goals are being pursued partly through the use of genetically mosaic (allophenic) mice produced by experimentally associating, throughout development, normal early embryo cells and embryo cells bearing a genetic defect or obtained from a strain that is susceptible to a particular malignancy. In other cases, mosaic individuals are being produced by injecting developmentally totipotent teratocarinoma stem cells into genetically marked normal blastocysts. There, the malignant cells become stably "normalized" and participate in embryogenesis. They contribute to somatic tissue differentiation and germ cells are also sometimes derived from the tumor lineage, thus giving rise to progeny. Therefore, these teratocarcinoma stem cells may be used as a means of producing mouse models of human genetic diseases. This is being done by first mutagenizing the cells in vitro and then subjecting them to specific selective procedures to obtain clones of totipotent cells with a known mutational defect; the mutant cells are next injected into blastocysts. Mosaic animals comprising some cells deficient in hypoxanthine phosphoribosyl transferase (the same X-linked lesion as in human Lesch-Nyhan disease) have been successfully produced in this way. These experiments will be continued, along with others in which the effects of other biochemically identified lesions on differentiation may be analyzed in vivo. Efforts are also being conducted to reverse other malignancies by introducing their stem cells into an appropriate location in normal embryos, where more orderly differentiation might occur.